agressive rooster to one of my hens, what to do?

I have 12 hens and one rooster and my rooster is aggressive towards only one of my hens. Should I get rid of him or put him in the crock-pot? This one hen will allow my rooster to mount him however when he does he pecks the you know what out of her head. She has been locked up for the past month healing (which she is fine now). However, about 2 weeks ago I let this hen out thinking she would be ok. Well, the first thing when I let her out the rooster went after her again, pecking her head. So, put her back in the pen by herself. He only does this to this one hen. I had another hen that I had to get rid b/c this same rooster did the same thing to her, only he pecked her down to her skull (she was a hen that wouldn't allow him to mount her). I don't understand why this rooster only picks on this one hen. The other hens allow him to breed them. I am thinking either get rid of him or put him in the crock pot. The thing is if I get rid of him what's to say he's not going to do this if he goes somewhere else. Plus, I allow all my chickens to free range and it's just not fair keeping this one locked up 24/7 b/c of this stupid rooster. What would you guys do?